[0001] This invention relates to circuits for filtering electrical signals and, more particularly,
to magnetically tuneable circuits for filtering high frequency electromagnetic signals
propagating in a waveguide. Specifically, the invention in one embodiment is directed
to magnetically tuneable, four-ferrite-sphere waveguide bandpass filters having high
off resonance isolation.
[0002] Generally, bandpass filters transmit electrical signals within a given frequency
range and reject electrical signals having frequencies which lie outside the given
frequency range. One known type of bandpass filter is a variable frequency bandpass
filter whose frequency passband is altered by controlling the reactance of the circuit
parameters of the filter. Such variable frequency bandpass filters are utilized, for
example, as preselectors for swept-frequency signal analyzers, such as the HP 8566B
or 8562A signal analyzer or the HP 71300A modular measurement system available from
Hewlett-Packard Company, Signal Analysis Division, Rohnert Park, California.
[0003] One type of variable frequency bandpass filter is the magnetically tuneable filter.
In this filter, the frequency passband is varied by controlling the current to an
electromagnet which tunes variable frequency resonator elements in the filter across
different frequency ranges. Known variable frequency resonator elements include hexagonal
ferrite spheres and yttrium-iron-garnet (YIG) spheres.
[0004] The advantage of utilizing hexagonal ferrite spheres, instead of YIG spheres, for
magnetically tuneable filters in the millimeter wave region is that they have a large
internal anisotropy field (H
a), which reduces the applied magnetic field needed to achieve resonance (f
res = 2.8 MHz/oersted (H
a + H
applied)). By reducing the required magnetic field, problems associated with electromagnet
heating, hysteresis, tuning linearity, and maximum tuning frequency can be reduced.
[0005] Iris-coupled, two-sphere, magnetically tuneable millimeter wave bandpass filters
fabricated in a waveguide (Fig. 1) utilizing hexagonal ferrite spheres are known.
See, for example, Matthaei, G., Young, L., and Jones, E.M.T., "Microwave Filters,
Impedance-Matching Networks, and Coupling Structures," Artech House, 1980, pp. 1040-1085;
Sweschenikow, J.A., Merinow, E.K., and Pollak, B.P., "Bandfilter aus Hexaferriten
im Mikrowellenbereich," Nachrichtechnik Elektronik, 26, 1976, pp. 262-264; and Nicholson,
D., "A High Performance Hexagonal Ferrite Tuneable Bandpass Filter for the 40-60 GHz
Region," 1985 IEEE MTT-S International Microwave Symposium Digest, pp. 229-232. These
filters have been demonstrated to filter across full waveguide bandwidths up through
W band, as shown in Figs. 2, 3, and 4.
[0006] The extension of the iris-coupled, two-ferrite-sphere waveguide bandpass filter
utilizing one electromagnet to a three- or four-sphere structure having increased
off resonance isolation has not, however, been satisfactorily achieved. The aforementioned
Sweschenikow,
et al., article discloses a three-sphere waveguide bandpass filter utilizing hexagonal
ferrite spheres (Fig. 5). The addition of the third sphere did not increase the insertion
loss significantly, but only improved the off resonance isolation by approximately
12 dB, while unfortunately spreading the electromagnet pole tips farther apart. In
contrast, Fjerstad, R.L., "Some Design Considerations and Realizations of Iris-Coupled
YIG-Tuned Filters in the 12-40 GHz Region," IEEE Trans. on Microwave Theory and Techniques,
Vol. MTT-8, No. 4, April, 1970, pp. 205-212, discloses a four-sphere waveguide bandpass
filter utilizing YIG spheres (Fig. 6). However, this four-YIG-sphere waveguide bandpass
filter has a limited bandwidth and poor off resonance isolation at high frequencies.
A broadband millimeter wave bandpass filter having high off resonance isolation preferably
utilizing hexagonal ferrite spheres is, therefore, needed.
[0007] One embodiment of the present invention provides a magnetically tuneable four-sphere
bandpass filter preferably utilizing hexagonal ferrite spheres in a waveguide, without
increasing the separation of electromagnet pole tips, that has high off resonance
isolation. The invention provides in one embodiment a magnetically tuneable bandpass
filter comprising four ferrite spheres as resonators and further preferably comprising
TE₁₀ rectangular waveguide as an input waveguide, output waveguide, and transfer waveguide.
The four spheres are configured as a pair of two-sphere filters. Preferably, the pair
of two-sphere filters comprises a first sphere in the input waveguide placed directly
over a first iris below which is positioned a second sphere in the transfer waveguide
in an over-under configuration and a third sphere in the output waveguide placed directly
over a second iris below which is situated a fourth sphere in the transfer waveguide
in another over-under configuration. The pair of two-sphere filters is connected
by the transfer waveguide, thereby allowing all four spheres to be situated under
the area of the surface of one electromagnet pole tip. All spheres preferably have
waveguide shorts directly behind them in the waveguides in which they reside.
[0008] This approach differs from the approach disclosed in the aforementioned Fjerstad
article where a four-YIG-sphere bandpass filter was constructed in a single waveguide
with axial irises, instead of a plural waveguide structure including a transfer waveguide
and over-under sphere configurations as provided by one embodiment of the invention.
The use of the over-under configuration of the spheres provides tighter coupling between
them compared to the side-to-side coupling reported previously for the four-YIG-sphere
waveguide bandpass filter. This decreases insertion loss and increases bandwidth.
[0009] The magnetically tuneable, four-ferrite-sphere waveguide bandpass filter in accordance
with one embodiment of the invention achieves similar performance to two cascaded
two-sphere filters without requiring two separate electromagnets. The bandpass filter
of the invention exhibits full-band performance in A band (26.5-40 GHz), Q band (33-50
GHz), U band (40-60 GHz), and V band (50-75 GHz). A series of four-sphere magnetically
tuneable bandpass filters fabricated in a waveguide utilizing hexagonal ferrite spheres
as tuning elements in accordance with one embodiment of the invention, operated in
A, Q, U, and V bands, has typical off resonance isolation greater than 70 dB and insertion
loss less than 13 dB.
[0010] The above and other features of the invention and the concomitant advantages will
be better understood and appreciated by persons skilled in the field to which the
invention pertains in view of the following description given in conjunction with
the accompanying drawings. In the drawings:
Fig. 1, comprising Figs. 1A and 1B, is a schematic drawing of a known configuration
for a magnetically tuneable two-sphere waveguide bandpass filter in plan (Fig. 1A)
and in cross-section (Fig. 1B);
Fig. 2 illustrates a typical response characteristic for the two-sphere waveguide
bandpass filter shown in Fig. 1;
Fig. 3 is a plot of insertion loss versus percent of frequency band for the two-sphere
waveguide bandpass filter shown in Fig. 1;
Fig. 4 is a plot of off resonance isolation versus percent of frequency band for the
two-sphere waveguide bandpass filter shown in Fig. 1;
Fig. 5 is a cross-sectional schematic drawing of a known configuration for a magnetically
tuneable three-sphere waveguide bandpass filter;
Fig. 6, comprising Figs. 6A and 6B, is a schematic drawing of a known configuration
for a magnetically tuneable four-YIG-sphere waveguide bandpass filter in plan (Fig.
6A) and in cross-section (Fig. 6B);
Fig. 7, comprising Figs. 7A and 7B, is a schematic drawing of a magnetically tuneable
four-sphere waveguide bandpass filter in accordance with one embodiment of the invention
in plan (Fig. 7A) and in cross-section (Fig. 7B);
Fig. 8 is an exploded view of one implementation of the four-sphere waveguide bandpass
filter shown in Fig. 7;
Fig. 9 illustrates a typical response characteristic for the four-sphere waveguide
bandpass filter shown in Fig. 7;
Fig. 10 is a diagram which shows transfer waveguide resonances for the four-sphere
waveguide bandpass filter shown in Fig. 7;
Fig. 11, comprising Figs. 11A and 11B, is a schematic drawing in plan (Fig. 11A) and
in cross-section (Fig. 11B) of a magnetically tuneable four-sphere waveguide bandpass
filter shown in Fig. 7 modified to additionally comprise lossy dielectric material
in the transfer waveguide;
Fig. 12 illustrates a typical response characteristic for the four-sphere waveguide
bandpass filter shown in Fig. 11;
Fig. 13, comprising Figs. 13A, 13B, and 13C, is a schematic drawing in plan (Fig.
13A) and in cross-section (Fig. 13B) of a modified magnetically tuneable four-sphere
waveguide bandpass filter similar to that shown in Fig. 7 but having offset spheres
and a shortened transfer waveguide to suppress cavity resonances, as well as a schematic
drawing in cross-section (Fig. 13C) having dielectric material added behind backshorts
of the transfer waveguide;
Fig. 14 illustrates a typical response characteristic for the four-sphere waveguide
bandpass filter shown in Fig. 13 which exhibits elimination of typical cavity modes;
Fig. 15 is a schematic drawing in plan of a magnetically tuneable four-sphere waveguide
bandpass filter having an external isolator in accordance with another embodiment
of the invention;
Fig. 16 is a schematic drawing in plan of an integrated bandstop filter in a bandpass
filter in accordance with a further embodiment of the invention;
Fig. 17, comprising Figs. 17A and 17B, is a schematic drawing of a magnetically tuneable
four-sphere ridge waveguide bandpass filter in plan (Fig. 17A) and cross-section (Fig.
17B) in accordance with an additional embodiment of the invention; and
Fig. 18, comprising Figs. 18A and 18B, illustrates a typical response characteristic
for the four-sphere waveguide bandpass filter shown in Fig. 7 (Fig. 18A) and the four-sphere
ridge waveguide bandpass filter shown in Fig. 17 (Fig. 18B) which exhibits a decrease
in insertion loss.
[0011] A schematic drawing of one embodiment of a four-sphere waveguide bandpass filter
in accordance with the invention, generally indicated by the numeral 10, is shown
in Fig. 7. The waveguide bandpass filter 10 comprises a pair of two-sphere waveguide
bandpass filters, including a first two-sphere waveguide bandpass filter 12 and a
second two-sphere waveguide bandpass filter 14, connected by a transfer waveguide
16. This allows all four spheres to fit under one electromagnet pole tip 18. Another
electromagnet having a pole tip 20 is preferably included for increasing the applied
magnetic field.
[0012] As shown in Figs. 7 and 8, the first two-sphere filter 12 comprises a first sphere
22 in an input waveguide 24. The first sphere 22 is placed directly over a first iris
26 in an iris plate 28 below which is positioned a second sphere 30 in the transfer
waveguide 16 in an over-under configuration. The second two-sphere filter 14 comprises
a third sphere 32 in an output waveguide 34. The third sphere 32 is placed directly
over a second iris 36 in the iris plate 28 below which is situated a fourth sphere
38 in the transfer waveguide 16 in another over-under configuration. Accordingly,
the input waveguide 24 and the output waveguide 34 overlie the transfer waveguide
16. The pair of two-sphere filters 12 and 14 is connected by the transfer waveguide
16, thereby allowing all four of the spheres 22, 30, 32, and 38 to be situated under
the area of the surface of one electromagnet pole tip 18 or between the electromagnet
pole tips 18 and 20. The spheres 22, 30, 32, and 38 preferably have waveguide shorts
directly behind them in the waveguides in which they reside to increase the magnetic
coupling of the energy in the input, output, and transfer waveguides 24, 34, and 16
to the spheres.
[0013] As shown in Fig. 8, the spheres 22, 30, 32 and 38 are preferably mounted on annular
dielectric holders 40 preferably glued circumferentially around the irises 26 and
36. Alternatively, the spheres 22, 30, 32, and 38 can be mounted on dielectric rods
(not shown) which are movable to allow adjustment of sphere position and rotation
with respect to the irises 26 and 36. Also, any combination of the above-described
mounting arrangements can be employed.
[0014] The spacing d of the two sets of spheres 22, 30 and 32, 38 from each other is approximately
one waveguide width, which becomes a small distance in the millimeter wave region.
This allows compact electromagnets to be utilized. The size of the spheres 22, 30,
32, and 38 in relation to the waveguide width and height and the sphere-to-sphere
separation (top to bottom) are set so as to give a maximally flat filter response
to avoid ripples in the frequency passband. The spheres 22, 30, 32, and 38 preferably
consist of barium ferrite crystals.
[0015] The input waveguide 24 and the output waveguide 34 are both preferably perpendicular
to the transfer waveguide 16. The input waveguide 24 and the output waveguide 34 are
kept at 90° angles to the transfer waveguide 16 to create magnetic field mode mismatches
to increase off resonance isolation.
[0016] The input, output, and transfer waveguides 24, 34, and 16 are all preferably reduced
in height between the electromagnet pole tips 18 and 20, as shown in Figs. 7B and
8. This reduces the current required for tuning. The input waveguide 24 and the output
waveguide 34 preferably have a linear taper to transition from reduced height under
the electromagnet pole tips 18 and 20 to standard height waveguide at connecting flanges
42.
[0017] Dielectric loading can be introduced into the input, output, and transfer waveguides
24, 34, and 16 by the incorporation of dielectric material 43, such as Rexolite, as
shown in dotted lines in Fig. 8. The advantage is that this allows utilization of
narrower waveguides and thus a smaller diameter electromagnet for a given frequency
range. As will be described in more detail later, this shifts the frequency passband.
A typical dimension for the dielectric material 43 in a 33-50 GHz waveguide bandpass
filter 10 to shift it to a 26.5-40 GHz waveguide bandpass filter can be, for example,
0.64 mm high by 2.5 mm wide Rexolite.
[0018] In operation, at the resonant frequency of the spheres 22, 30, 32, and 38, energy
is taken from the input waveguide 24 and coupled into the first sphere 22 in the input
waveguide and is then coupled through the first iris 26 under the sphere 22 to the
second sphere 30 directly below it in the transfer waveguide 16, which reradiates
the energy down the transfer waveguide. The energy reradiated down the transfer waveguide
16 is coupled into the fourth sphere 38 in the transfer waveguide and is then coupled
through the second iris 36 to the third sphere 32 directly above it in the output
waveguide 34. The third sphere 32 in the output waveguide 34 reradiates the energy,
which then travels down the output waveguide. Off the resonant frequency of the spheres
22, 30, 32, and 38, the small diameter of the irises 26 and 36 substantially prevents
energy from coupling from the input waveguide 24 to the output waveguide 34.
[0019] Based on a comparison of the four-sphere waveguide bandpass filter 10 to two cascaded
two-sphere filters, the off resonance isolation is expected to be double (in dB) the
value for a pair of cascaded two-sphere filters. The insertion loss of the four-sphere
waveguide bandpass filter 10 is also expected to be about double (in dB) the amount
for a pair of cascaded two-sphere filters. Advantageously, however, by eliminating
one set of input and output transitions present in a pair of cascaded two-sphere filters,
the insertion loss is expected to be .5-1 dB less than a simple doubling.
[0020] A four-ferrite-sphere, magnetically tuneable waveguide bandpass filter 10 in accordance
with one embodiment of the invention was implemented with WR-15 waveguide and utilizing
doped BaFe₁₂O₁₉ spheres for the variable frequency resonator elements and tested in
the 50-75 GHz region. Insertion loss results for the entire region and typical off
resonance isolation results demonstrate that the expected performance was achieved.
Fig. 9 illustrates a typical four-ferrite-sphere waveguide bandpass filter 10 response
in V band. It can be seen that the insertion loss is slightly less than twice that
(in dB) expected of a pair of cascaded two-sphere V band filters (Figs. 2 and 3).
The off resonance isolation is about twice (in dB) that of a pair of cascaded two-sphere
V band filters. The results produced by the filters are similar for the A, Q, and
U bands.
[0021] Utilizing a transfer waveguide 16 to connect the pair of two-sphere filters 12 and
14 allows the entire four-sphere waveguide bandpass filter 10 to be placed under one
electromagnet pole tip 18, thereby decreasing the energy needed to tune the filter
by one-half compared to a pair of cascaded two-sphere filters each with its own electromagnet
and connected by a longer length of waveguide. By utilizing a four-sphere waveguide
bandpass filter 10, which has two irises 26 and 36 to reject out-of-band energy, instead
of a two-sphere filter with only one iris, the out-of-band rejection of energy is
greatly increased, thereby leading to better performance in preselectors for swept-frequency
signal analyzers and other uses.
[0022] Referring again to Fig. 7, the transfer waveguide 16 with a backshort at both ends
forms a waveguide resonator. At the cavity resonant frequency, energy can be coupled
to an undesirable extent from the input waveguide 24 to the output waveguide 34 through
the irises 26 and 36. Cavity resonances in the transfer waveguide 16 are shown for
V band in Fig. 10. Calculations indicated that a λg/2 mode would occur at 48.9 GHz
and a λg mode at 69.3 GHz. At these frequencies, the length of the transfer waveguide
16 acts as a bandpass filter, degrading the off resonance isolation.
[0023] As shown in Fig. 9, the λg cavity mode which was expected to appear at 69.3 GHz actually
appears at 67.5 GHz due to dielectric loading effects at the spheres 22, 30, 32, and
38. The typical λg/2 mode is below the bottom of the band (50 GHz) as expected.
[0024] This cavity resonance can be de-Q'd and suppressed by introducing a small amount
of lossy dielectric material which has the same loss in both directions. Also, the
cavity can be de-Q'd by introducing a high resistivity metal plating on the transfer
waveguide 16 or composite material, such as polyiron, in the cavity itself. Alternatively,
by introducing loss only in the reverse direction with an appropriately placed resonance
isolator material, the cavity can be de-Q'd with negligible increase in filter insertion
loss. See, for example, Taft, D.R., Harrison, G.R., Hodges, Jr., L.R., "Millimeter
Resonance Isolators Utilizing Hexagonal Ferrites," IEEE Trans. on Microwave Theory
and Techniques, Vol. MTT-11, No. 5, September, 1963, pp. 346-350, the disclosure of
which is hereby incorporated by reference in its entirety.
[0025] Preferably, the excess feedthrough caused by the λg resonance is greatly decreased
by introducing a small amount of lossy dielectric material in the transfer waveguide
16. As shown in Fig. 11, this loss can be introduced either by an attenuating vane
44 in the transfer waveguide 16 or by placing a thin (approximately 50 µm) sheet of
dielectric, such as Kapton, at the backshorts in the transfer waveguide, as shown
in Fig. 13C. As shown in Fig. 12, introduction of 1-2 dB of loss in the transfer waveguide
16 yields about 15-20 dB of attenuation in the cavity mode induced feedthrough.
[0026] Use of a reciprocally lossy material in the transfer waveguide 16 to suppress the
cavity resonance induced feedthrough from the input to the output decreases off resonance
feedthrough at the cavity resonance frequency, while introducing only a small increase
in frequency passband insertion loss. Alternatively, use of loss in only the reverse
direction in the cavity of the transfer waveguide 16 produces a very large decrease
in the cavity resonance induced feedthrough with only a negligible increase in passband
insertion loss.
[0027] In addition to the typical cavity modes which can occur many GHz away from the frequency
passband, there are also λg/2 and λg modes which are perturbed by the large permeability
(both positive and negative), which occurs near resonance, and tend to follow within
a few hundred MHz of the frequency passband as it is tuned. With no loss introduced
in the transfer waveguide 16, these perturbed cavity modes can produce ripples in
the frequency passband. With the introduction of loss or an appropriately placed resonance
isolator material, these irregularities are greatly reduced. If the degradation in
off resonance isolation due to the typical λg cavity mode is, however, unacceptable,
even after it has been reduced by 15-20 dB with the introduction of loss in the transfer
waveguide 16, for example, it can be eliminated completely by the following technique.
[0028] As shown in Fig. 13, by offsetting the spheres 22, 30, 32, and 38 from the center
of the input waveguide 24 and output waveguide 34 so that they are slightly closer
to each other, the transfer waveguide 16 can be shortened enough to push the λg mode
out the top of the band. Shortening the transfer waveguide 16 brings the λg/2 mode
in near the bottom of the band, but it's frequency can be reduced below band again
by placing a piece of dielectric material midway between the spheres 30 and 38 in
the transfer waveguide. The point midway between the spheres 30 and 38 in the transfer
waveguide 16 is an E field null for the λg mode, so that it's frequency is not affected,
and both typical modes are now out of band. An example of this technique is shown
in a Q band filter response in Fig. 14. In the case of a shortened transfer waveguide
16 with off-center spheres 22, 30, 32, and 38, the introduction of dielectric material
(not shown), such as Rexolite, into the input, output, and transfer waveguides 24,
34, and 16, including proper placement slightly off center in the input and output
waveguides, can keep the magnetic fields over the irises 26 and 36 parallel to backshorts
for good off resonance isolation in short transfer waveguide bandpass filters.
[0029] A performance summary for the four-sphere waveguide bandpass filter 10 in accordance
with the invention is given below in Table I showing the similar performance obtained
in the different bands. All results are for filters with loss introduced in the transfer
waveguide 16 for cavity mode suppression. The four-sphere filter parameters are as
follows.
TABLE I
Band (GHz) |
Insertion Loss (dB) |
O.R.I. (Typical) |
3dB Bandwidth (MHz) |
Electromagnet Power (WATTS) |
Minimum |
Maximum |
Minimum |
Maximum |
Minimum |
Maximum |
A (26.5-40) |
11.0 |
14.0 |
>75 dB |
120 |
200 |
.27 |
4.7 |
Q (33-50) |
10.0 |
13.0 |
>75 dB |
180 |
260 |
.28 |
7.1 |
U (40-60) |
8.0 |
12.0 |
>75 dB |
180 |
270 |
.28 |
8.4 |
V (50-75) |
8.0 |
12.0 |
>70 dB |
190 |
300 |
.35 |
12.0 |
[0030] As shown in Fig. 15, the pair of two-sphere filters 12 and 14 under one electromagnet
pole tip 18 is provided with a transfer waveguide 16' external to the electromagnet
as a connection between the filters. The advantages are that with an external waveguide
connection between the pair of two-sphere filters 12 and 14, a commercially available
full-band isolator 46 can be utilized to completely suppress the transfer waveguide
resonances. This configuration is advantageous when any cavity mode perturbation on
the frequency passband is undesirable or the addition of dielectric material or loss
to a short transfer waveguide 16 sufficient to suppress modes to the desired extent
becomes prohibitive in terms of insertion loss. Alternatively, an amplifier can be
inserted between the filters 12 and 14 to cancel out the loss of the second filter
14, as well as to provide isolation between the two filters.
[0031] Fig. 16 shows an extra sphere 48 or spheres positioned relative to the backshort
of at least one of the input, output, and transfer waveguides 24, 34, and 16, for
example, in the transfer waveguide, so that they act as bandstop filters integrated
into the four-sphere waveguide bandpass filter 10. Hexagonal ferrite spheres can have
greatly different resonant frequencies, and a sphere or spheres can be advantageously
selected that resonate at a frequency offset, in relation to the spheres 22, 30, 32,
and 38, corresponding to that frequency at which an extra large attenuation is desired.
This allows filter skirts and general off resonance isolation to be tailored in ways
that would otherwise be physically cumbersome (extra electromagnet required) or impossible.
[0032] Finally, as shown in Fig. 17, a ridge waveguide having ridges 50 positioned over
the spheres 22, 30, 32, and 38 in the input, output, and transfer waveguides 24, 34,
and 16 increases magnetic field coupling. The advantages are that by increasing the
magnetic field coupling from the input, output, and transfer waveguides 24, 34, and
16 to the spheres 22, 30, 32, and 38, insertion loss is decreased in a waveguide bandpass
filter, as shown in Fig. 18, and the bandwidth in waveguide bandpass and bandstop
filters is broadened.
1. A magnetically tunable waveguide bandpass filter (10), for use in a variable intensity
magnetic field, characterised in that it comprises an input and an output waveguide
(24,34) connected by a transfer waveguide (16); a first two-sphere waveguide bandpass
filter (12) comprising a first sphere (22) in the input waveguide placed above a first
Iris (26) below which is positioned a second sphere (30) in the transfer waveguide;
and a second two-sphere waveguide bandpass filter (14) comprising a third sphere (32)
in the output waveguide placed above a second Iris (36) below which is situated a
fourth sphere (38) in the transfer waveguide; and wherein the two-sphere filters are
connected by the transfer waveguide.
2. A waveguide bandpass filter (10) according to claim 1 wherein the first sphere
(22) is positioned directly above the first Iris (26) and the third sphere (32) is
positioned directly above the second Iris (36).
3. A waveguide bandpass filter (10) according to claim 1 or 2, wherein all of the
spheres (22, 30, 32, 38) are centred on a centrally positioned, vertical plane bisecting
the length of input waveguide (24) or a centrally positioned, vertical plane bisecting
the length of the output waveguide (34).
4. A waveguide bandpass filter (10) according to claim 1 or 2; wherein the first and
second spheres (22, 30) are laterally displaced from a vertical plane bisecting the
length of the input waveguide (24) and the third and fourth spheres (32, 38) are laterally
displaced from a vertical plane bisecting the output waveguide (34); and wherein the
direction of the displacements is such that the separation between the first and second
two-sphere waveguide bandpass filters (12, 14) is reduced.
5. A waveguide bandpass filter (10) according to any preceding claim further comprising
lossy dielectric material (43) situated in the transfer waveguide (16).
6. A waveguide bandpass filter (10) according to any preceding claim wherein one or
more of the waveguides (16, 24, 34) comprises dielectric loading means disposed therein.
7. A waveguide bandpass filter (10) according to any preceding claim, wherein the
spheres (22, 30, 32, 38) comprise barium ferrite crystals.
8. A waveguide bandpass filter (10) according to any preceding claim, wherein the
input and output waveguides (24, 34) are both at right angles to the transfer waveguide
(16).
9. A waveguide bandpass filter (10) according to any preceding claim wherein the ends
of the input and output waveguides (24, 34) that are adjacent to the transfer waveguide
(16) are tapered.
10. A waveguide bandpass filter (10) according to any preceding claim, wherein the
spheres (22, 30, 32, 38) are mounted on annular dielectric holders (40) positioned
circumfentially around the irises (26, 36).
11. A waveguide bandpass filter (10) according to any preceding claim, wherein all
of the waveguides (16,24,34) are TE₁₀ waveguides.
12. A waveguide bandpass filter (10) according to any preceding claim, further comprising
a ridge waveguide (50) positioned over the spheres (22,30, 32, 38).
13. A waveguide bandpass filter (10) according to any preceding claim, further comprising
one or more additional spheres positioned in the waveguides (16, 24, 34).
14. An apparatus comprising means (20) for producing a magnetic field of variable
intensity and a waveguide bandpass filter (10) according to any preceding claim.